The present invention relates to a laser apparatus, and particularly to a laser apparatus suitable for annealing treatment performed for improvement of characteristics during the manufacturing of TFTs (Thin Film Transistors) used as an active device for controlling current and a switching device of each pixel in an active matrix display employing liquid crystal or an organic electroluminescence (EL) device, for example.
Generally, an active matrix display has a large number of pixels arranged in a matrix manner, and displays an image by controlling light intensity of each of the pixels according to given brightness information.
When liquid crystal is used as electro-optical material, transmissivity of each pixel is changed according to voltage written to the pixel.
An active matrix display using an organic EL device as electro-optical material basically operates in the same manner as when liquid crystal is used as electro-optical material.
In a liquid crystal display (LCD), for example, a TFT is used as a switching device of each pixel to apply voltage to a liquid crystal layer on a display electrode. The TFT controls alignment of the liquid crystal and thereby controls transmission of backlight from the back side of a glass substrate.
Liquid crystal displays and the like use quartz glass having resistance to a high heat temperature of 1000xc2x0 C. as the glass substrate. Since the quartz substrate is expensive, however, inexpensive glass having resistance to a low heat temperature of 600xc2x0 C. has recently been used.
Thus, process temperatures in TFT fabrication are controlled below the heat temperature that the substrate can resist. Polycrystalline silicon (polysilicon) has a carrier mobility higher than that of amorphous silicon (a-Si) film by about two orders of magnitude, and therefore is suitable for use in a large display having a high operating speed and having a large number of pixels.
When polysilicon is to be formed on a glass substrate, a method is used which anneals a-Si film by laser light to recrystallize the a-Si film into polysilicon film, to avoid thermal deformation and the like of the glass substrate.
Conventionally, a XeCl excimer laser having an oscillation wavelength of 308 nanometers (nm) is used in a laser annealing apparatus for carrying out the annealing treatment.
According to known materials of Lambda Physik, the fluence required to anneal a TFT using the XeCl excimer laser having the oscillation wavelength of 308 nm is a few hundred mJ/cm2; the average output power is 200 W; the repetition frequency is 300 Hz; and the pulse width is about 20 nanoseconds (ns).
This makes it possible to obtain optical energy sufficient to melt an a-Si film surface. Incidentally, the silicon surface is melted by annealing to 1420xc2x0 C. or higher.
Although the excimer laser can achieve a high output in a short-wavelength range, however, the excimer laser has disadvantages of having a large apparatus size and lacking in stability of output pulses.
As a result, the annealing apparatus becomes large and lacks in stability of annealing processing. Thus, there has been a desire for development of a laser apparatus having a smaller size and producing stable output pulses.
It is accordingly an object of the present invention to provide a laser apparatus that can be miniaturized and can stabilize output pulses, and accordingly makes it possible to miniaturize and stabilize annealing apparatus and the like.
In order to achieve the above object, according to a first aspect of the present invention, there is provided a laser apparatus including: a laser light source for emitting reference laser pulse light having a predetermined wavelength and a predetermined pulse width; a plurality of optical fibers having different propagation delay characteristics for propagating light; light dividing means for dividing the reference laser pulse light emitted from the laser light source into a plurality of pieces of light to propagate each of the divided pieces of reference laser pulse light through one of the plurality of optical fibers; and light combining means for successively disposing the divided pieces of reference laser pulse light propagated through the plurality of optical fibers and emitted from the plurality of optical fibers in parallel with each other and emitting laser pulse light having a pulse width greater than the pulse width of the reference laser pulse light.
Also, in the laser apparatus according to the first aspect of the present invention, the light combining means includes: wavelength changing means for changing wavelength of the divided pieces of reference laser pulse light emitted from the plurality of optical fibers to a wavelength shorter than the predetermined wavelength; and an optical system for successively disposing the plurality of pieces of laser pulse light changed in wavelength by the wavelength changing means in parallel with each other and emitting laser pulse light having a pulse width greater than the pulse width of the reference laser pulse light.
In addition, in the laser apparatus according to the first aspect of the present invention, the wavelength changing means includes at least one nonlinear optical crystal for generating an n-order harmonic (n is an integer of two or more) on the basis of incident light.
Preferably, the wavelength changing means includes: a first nonlinear optical crystal for receiving the divided pieces of reference laser pulse light emitted from the plurality of optical fibers, generating a plurality of second harmonics, and emitting the plurality of divided pieces of reference laser pulse light and the plurality of second harmonics; and a second nonlinear optical crystal for generating third harmonics on the basis of the plurality of divided pieces of reference laser pulse light and the plurality of second harmonics emitted from the first nonlinear optical crystal; and the optical system successively disposes the plurality of third harmonics emitted from the second nonlinear optical crystal in parallel with each other and emits laser pulse light having a pulse width greater than the pulse width of the reference laser pulse light.
Moreover, in the laser apparatus according to the first aspect of the present invention, the plurality of optical fibers are each set at a different propagation length so that the pieces of laser pulse light are sequentially emitted with a propagation delay time corresponding to the pulse width of the reference laser pulse light.
According to a second aspect of the present invention, there is provided a laser apparatus including: a laser light source for emitting reference laser pulse light having a predetermined wavelength and a predetermined pulse width; a plurality of optical fiber amplifiers having different propagation delay characteristics for propagating light for amplifying the propagating light with a gain corresponding to intensity of excitation light supplied thereto; excitation light supplying means for supplying the excitation light to the plurality of optical fiber amplifiers; light dividing means for dividing the reference laser pulse light emitted from the laser light source into a plurality of pieces of light to propagate each of the divided pieces of reference laser pulse light through one of the plurality of optical fiber amplifiers; and light combining means for successively disposing the divided pieces of reference laser pulse light propagated through the plurality of optical fiber amplifiers and emitted from the plurality of optical fiber amplifiers in parallel with each other and emitting laser pulse light having a pulse width greater than the pulse width of the reference laser pulse light.
Also, in the laser apparatus according to the second aspect of the present invention, the light combining means includes: wavelength changing means for changing wavelength of the divided pieces of reference laser pulse light emitted from the plurality of optical fiber amplifiers to a wavelength shorter than the predetermined wavelength; and an optical system for successively disposing the plurality of pieces of laser pulse light changed in wavelength by the wavelength changing means in parallel with each other and emitting laser pulse light having a pulse width greater than the pulse width of the reference laser pulse light.
In addition, in the laser apparatus according to the second aspect of the present invention, the wavelength changing means includes at least one nonlinear optical crystal for generating an n-order harmonic (n is an integer of two or more) on the basis of incident light.
Preferably, the wavelength changing means includes: a first nonlinear optical crystal for receiving the divided pieces of reference laser pulse light emitted from the plurality of optical fiber amplifiers, generating a plurality of second harmonics, and emitting the plurality of divided pieces of reference laser pulse light and the plurality of second harmonics; and a second nonlinear optical crystal for generating third harmonics on the basis of the plurality of divided pieces of reference laser pulse light and the plurality of second harmonics emitted from the first nonlinear optical crystal; and the optical system successively disposes the plurality of third harmonics emitted from the second nonlinear optical crystal in parallel with each other and emits laser pulse light having a pulse width greater than the pulse width of the reference laser pulse light.
Moreover, in the laser apparatus according to the second aspect of the present invention, the plurality of optical fiber amplifiers are each set at a different propagation length so that the pieces of laser pulse light are sequentially emitted with a propagation delay time corresponding to the pulse width of the reference laser pulse light.
Furthermore, in the laser apparatus according to the second aspect of the present invention, intensity of the excitation light supplied to the plurality of optical fiber amplifiers is set at a desired value for each of the plurality of optical fiber amplifiers.
According to the present invention, the laser light source emits reference laser pulse light having a predetermined wavelength, for example a near-infrared wavelength of 914 nm and a pulse width of 0.5 ns to the light dividing means.
The light dividing means divides the reference laser pulse light entered therein into a plurality of pieces of light to propagate the divided pieces of reference laser pulse light through the plurality of optical fiber amplifiers, for example.
Each of the optical fiber amplifiers is supplied with the excitation light by the excitation light supplying means, for example. Each of the divided pieces of reference laser pulse light being propagated through the optical fiber amplifiers is amplified with an induction gain corresponding to intensity of the excitation light to compensate for a reduction in optical power caused by dividing the reference laser pulse light, and then the divided pieces of reference laser pulse light are emitted from the other end surfaces of the optical fiber amplifiers to the light combining means.
The plurality of optical fiber amplifiers are provided with propagation delay characteristics different from each other for the divided pieces of reference laser pulse light, or the propagating light.
For example, the plurality of optical fiber amplifiers are each set at a length such that the pieces of laser pulse light are sequentially emitted with a propagation delay time corresponding to the pulse width of the reference laser pulse light.
Thus, the plurality of divided pieces of reference laser pulse light are emitted from the plurality of optical fiber amplifiers to the light combining means with a timing relation representing a shift of 0.5 ns, for example.
In the light combining means, the first nonlinear optical crystal, for example, changes the wavelength, which is 914 nm, of each of the plurality of divided pieces of reference laser pulse light incident thereon on the basis of nonlinear polarization to thereby generate second harmonics having a wavelength of 457 nm.
The plurality of second harmonics generated by the first nonlinear optical crystal and the plurality of divided pieces of reference laser pulse light are entered into the second nonlinear optical crystal.
The second nonlinear optical crystal then performs sum frequency mixing of the plurality of divided pieces of reference laser pulse light having the wavelength of 914 nm and the plurality of second harmonics having the wavelength of 457 nm entered therein, thereby generates a plurality of third harmonics having a wavelength of 305 nm, and then emits the plurality of third harmonics to the optical system.
Then, the optical system successively disposes the plurality of third harmonics in parallel with each other and emits laser pulse light having a pulse width greater than the pulse width of the reference laser pulse light.